feat(ui): group federated specs by project on the Specs page

[silversurfer562]

Summary

• Wrap the Specs page table in per-project <details> blocks driven by spec.project (plumbed by feat(specs): federated multi-root spec listing + coverage config #30). The section matching the active workspace expands by default; others collapse.
• Drop the single specs_root meta-line in favor of a "Reading from N root(s)" hint when multiple roots are configured.
• Only project is surfaced — root (filesystem noise) and collision (the natural "same slug under two sections" signal is now visible structurally) are not rendered.

Federation is now visible to the user instead of disappearing into a flat list of ~28 rows.

Approved proposal: attune-ai/docs/specs/spec-viewer-ia/proposal.md

Out of scope (deferred per proposal)

• Per-row phase tracker (R · D · T 3-cell idea from §6) — needs a backend change to _scan_feature and deserves its own review.
• Filtering, sorting, recency.

Test plan

• pytest sidecar/tests/test_cowork_pages.py (20 passed, including new test_specs_page_groups_by_project)
• ruff check clean on changed .py files
• Manual: verify two-project federation (attune-ai + attune-gui) renders two collapsible groups, workspace project open by default
• Manual: verify empty-state ("No specs found") still renders when zero roots configured

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